What testing content is required in the mill roll manufacturing process?
During the manufacturing process, the rolls have to undergo multiple cold and hot processing processes, and the inspections involved include:
1) Mechanical dimension, shape and position tolerance inspection and surface quality inspection of each machining process;
2) Flaw detection, including ultrasonic flaw detection, including surface wave flaw detection and radial ultrasonic flaw detection, surface coloring flaw detection, magnetic particle flaw detection and eddy current flaw detection;
3) High and low magnification detection of materials, such as acid leaching test, metallographic detection
4) Mechanical performance testing.
5) Chemical composition detection
6) Hardness testing
What is the high and low magnification detection of the mill roll?
High-magnification inspection means that after the roll blank is sampled according to regulations, it is prepared for metallographic test, and then according to the requirements of the standard, under the condition of corrosion and non-corrosion, magnify a certain multiple under the microscope to observe its structure.
The content that can be detected includes: the level of inclusions, the level of carbide network, the level of liquid carbide, quenching grain size, banded structure and other microstructure characteristics.
Low-magnification inspection is to sample the roll or blank according to the regulations, polish the surface, then pickle and corrode, and observe with the naked eye or a conventional magnifying glass after cleaning to detect the segregation and porosity of the roll and other defects such as slag inclusions, shrinkage cavities, and cracks.
What is mill roll tempering and what is the role of tempering?
Quenching and tempering is the general term for the whole process of heating the roll blank according to a certain heat treatment process in a heat treatment furnace after rough machining, and tempering after heat preservation and quenching. Quenching and tempering can play the following roles:
(1) Make the roll core and roll neck achieve good mechanical properties and meet the needs of rolling conditions. The quenched and tempered roll material has an increased yield ratio, which can effectively improve the fatigue life of the roll.
(2) Meet the technical requirements of the designed roll neck. The roll bearing gear has a certain hardness requirement, which can be achieved through quenching and tempering, and additional heat treatment is required for special requirements.
(3) Prepare the structure for the final heat treatment of the roll. The high-temperature tempering after tempering and quenching causes some carbides to disperse and precipitate, and these carbides are more easily dissolved in the surface quenching of the subsequent final heat treatment.
(4) It can improve the matrix structure, especially the distribution of carbides.
What is the main cause of mill roll spalling?
The harm of spalling to the cold rolling site has been widely known by the cold rolling mill site and the grinding roll room. There are various causes of spalling, mainly including:
(1) Spalling caused by thermal shock cracks on the surface of the roll. Surface cracks or soft areas are generated by rolling accidents during the use of the roll. If they are not removed by grinding in time, they will expand into spalling cracks during use.
(2) Spalling caused by fatigue cracks in the subsurface of the roll. The main reason is that the contact fatigue of the roll forms subsurface cracks.
(3) Spalling caused by the metallurgical quality of the roll. Such as inclusions and abnormal carbides as fatigue-induced spalling.
What method is used to test the hardness of the mill roll?
Hardness is an important index to evaluate the quality of roll manufacturing and use. Conventional hardness testing mostly uses Rockwell and Vickers hardness testing methods, which belong to the press-in hardness testing method, and are characterized by good stability. Rockwell and Vickers hardness testing is usually easy to perform on small parts. However, for large forged steel rolls, due to the large volume of the workpiece, it is difficult to lift and place, so the Shore and Leeb hardness testing methods are widely used.
What should be paid attention to in mill roll hardness testing?
The hardness of the roll is an indirect physical value, and its level is affected by the internal structure of the roll itself. Such as the hardness of the matrix of the roll material, the type and quantity of carbides in the roll material, the residual stress of the roll, and so on. At the same time, since the Shore and Leeb hardness tests commonly used in roll hardness testing are both rebound hardness tests, they are greatly affected by other factors such as the state of the testing instrument and the psychological factors of the operator. Therefore, both the manufacturing and use departments of rolls need to be equipped with special personnel to be responsible for hardness testing. Pay attention to the selection of hardness tester, and the comparison relationship with other hardness should be stable. At the same time, attention should be paid to frequent inspection and proofreading of hardness testing instruments and standard test blocks. Enterprises with conditions can promote the use of standard rolls for proofreading of hardness testers.
What is the role of surface metallographic testing?
Usually metals are polycrystalline mixtures of several crystals. For example, for the commonly used steel types of forged steel rolls, the roll body is composed of ferrite and carbides before quenching, and the ferrite and some carbides are transformed into austenite by heating at high temperature during quenching. At the same time, a small amount of undissolved carbides are retained, and after quenching, martensite and a small amount of carbides are obtained. The morphology and quantity of the obtained structures are different due to different technological means. And these organizations and differences are microcosmic, and cannot be seen by the naked eye. Metallographic analysis is to use a metallographic microscope to observe and record a specially prepared metal sample to obtain information on material, process and performance. Since the roll cannot be destructively tested, it is usually polished, polished and acid-impacted directly on the test part of the roll body, and then observed with a special on-site microscope, which is called on-site metallographic testing.
How does the enterprise that uses the rolls carry out the purchase check and acceptance inspection of the rolls?
As far as the overall quality of the roll is concerned, its intrinsic quality has a great influence on the use of the roll. The key to internal quality control is the quality control in the manufacturing process. Therefore, in the purchase of rolls, it is first necessary to confirm the manufacturing capacity and quality system of the roll supplier to ensure that the quality of the entire production process is in a controlled state. At the same time, it is necessary to fully communicate with the manufacturer to reach reasonable technical conditions. Then there is the acceptance and use evaluation of the rolls. The qualified roll users can check the surface hardness, ultrasonic flaw detection, surface metallography and machining dimensions of the rolls according to their own equipment and personnel conditions. You can also entrust a professional testing company to carry out testing and analysis. For most users, it is difficult to carry out the above testing. It is recommended to test the key dimensions and the hardness of the roll body.
What are the common quality problems of mill rolls in use?
① Pit: The quality of the strip steel welds is not good, and foreign matter enters.
②Roll sticking: Excessive local reduction, fragmentation, piled steel, folding, poor strip shape or low surface hardness of the roll body.
③ Cracks: Overheating defects such as sticking rolls cause excessive local pressure, poor cooling, and coarse debris on the surface of the rolls.
④ Roller roll: the pressure at both ends of the roll body is inconsistent, the strip steel is wave-shaped, deviation, and the surface hardness of the roll body is low.
⑤Large-area stripping: due to the microcracks on the roll body, it is caused by the development of secondary fatigue cracks under the action of cyclic rolling stress.
⑥ Flesh loss: There are micro-cracks on the surface of the roll body, severe overheating on the surface of the roll body, and large inclusions on the surface of the roll body. Under the action of rolling stress, fatigue cracks develop into meat loss.
⑦ Annular cracks on both sides of the roll body: During rolling, the two ends are severely unevenly pressed, and the quenching quality of the roll is not good.
⑧Working roll bearing holding roll: the bearing is not well lubricated, oil is not added in time, the bearing seal is not good, and the two ends of the roll body are pressed down severely.
⑨Broken roll: The residual tensile stress inside the roll is large, there are defects such as cracks in the center of the roll, serious pressure deviation at both ends of the roll body, and the influence of external temperature, the internal stress of the roll increases sharply.
What are the common types of damage to backup rollers during use?
There are usually five categories:
①Uneven degree of chilling and hardness of the roller surface causes indentation;
②Roll sticking due to broken belt and uneven cooling;
③The rolling time is too long, and the fatigue of the chilled layer on the roll surface causes cracks;
④The surface cracks expand, and the fatigue cracks are not completely worn out, resulting in peeling off;
⑤The work roll does not move the back-up roll and causes vibration marks and arc grooves on the roll body.
